Compliant probe apparatus

ABSTRACT

A mechanically compliant probe for electrically connecting to contact pads on microelectronic devices. The probe is used for burn-in of integrated circuits at the wafer level. Additional applications include probe cards for testing integrated circuits and sockets for flip-chips. One embodiment of the probe includes a probe tip ( 81 ) which is held on an extension arm ( 82 ) projecting laterally from an elongated flat spring ( 83 ). The spring is supported above a substrate ( 89 ) by posts ( 85 ) such that the probe tip moves vertically in response to a contact force on the probe tip. Deflection of the probe tip is compliantly limited by bending and torsional flexure of the sheet spring. Mechanical compliance of the tip allows arrays of the probe to contact pads on integrated circuits where the pads are not precisely planar.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is related to a co-pending application Ser. No.09/563,365 entitled “CONNECTOR APPARATUS,” filed contemporaneouslyherewith by the same inventor.

FIELD OF THE INVENTION

This invention relates to a compliant probe apparatus. In particular,this invention relates to the burn-in and testing of microelectronicdevices, and specifically to contact assemblies used for connectingelectrical signals to integrated circuits during burning-in and testingof individual chips and of full wafers.

BACKGROUND OF THE INVENTION

Microelectronic devices are subjected to a series of test proceduresduring the manufacturing process in order to verify functionality andreliability. Prior art testing procedures conventionally include waferprobe testing, in which microelectronic device chips are tested todetermine operation of each chip before it is diced from the wafer andpackaged. Prior art probe cards are built of long cantilever wires thatare used to test one or several chips at a time at the wafer level.

Typically, not all chips on a wafer are found to be operable in thewafer probe test, resulting in a yield of less than 100% good devices.The wafer is diced into individual chips and the good chips areassembled into packages. The packaged devices are dynamically burned-inby loading into sockets on burn-in boards and electrically operating ata temperature of from 125° C. to 150° C. for a burn-in period of 8 to 72hours in order to induce any defective devices to fail. Burn-inaccelerates failure mechanisms that cause infant mortality or earlyfailure of the devices, and allows these defective devices to bescreened out by a functional electrical test before they are usedcommercially.

A full functional test is done on packaged devices, which are operatedat various speeds in order to categorize each by maximum speed ofoperation. Testing discrete packaged devices also permits elimination ofany devices that failed during the burn-in process. Burn-in and test ofpackaged devices is accomplished by means of sockets specially suited tothe burn-in conditions and to high speed testing respectively. As aresult, conventional manufacturing processes are expensive and timeconsuming because of repeated handling and testing of individualdiscrete devices through a lengthy set of steps that adds weeks to thetotal manufacturing time for the device.

A considerable advantage in cost and in process time can be obtained byburn-in and test of the wafer before it is diced into discrete devices.Additional savings can be obtained by fabricating chip size packages oneach device on a wafer before the wafer is diced into discrete devices.A considerable effort has been expended by the semiconductor industry todevelop effective methods for wafer level packaging, burn-in and test inorder to gain benefits of a greatly simplified and shortened process formanufacturing microelectronic devices. In order to reap these benefits,it is necessary to provide means to burn-in and speed test chips beforethey are diced from the wafer into individual discrete devices.

Conventional cantilever wire probes, however, are not suited to burn-inand speed testing of devices on the wafer. Cantilever wire probes aretoo long and bulky to allow simultaneous contact to all of the deviceson a wafer, as required for simultaneous burn-in of all of the deviceson the wafer. In addition, long cantilever wire probes are not suitablefor functional testing of high-speed devices, among other things,because of a high self and mutual inductance of the long, parallel wirescomprising the probes.

A small, high-performance probe that can be made at low cost is requiredfor practical application of wafer burn-in and test procedures. To beuseful for wafer burn-in and test, the desired probes must reliablycontact all of the pads on the devices under test while they are on theundiced wafer. Probes for contacting the wafer must also provideelectrical contact to pads on devices even, and especially, where thepads vary in height on the surface of the wafer. In addition, the probesmust break through any oxide layers on the surface of the contact padsin order to make a reliable electrical contact to each pad. Manyapproaches have been tried to provide a cost-effective and reliablemeans to probe wafers for burn-in and test, without complete success.

The prior art reveals a number of attempts that have been tried toprovide small, vertically compliant probes for reliably contacting thepads on devices on a wafer. According to the invention represented byU.S. Pat. No. 4,189,825, a cantilever probe is provided for testingintegrated circuit devices. In FIG. 1, cantilever 28 supports sharp tips26 above aluminum contact pads 24 on a chip 23. A compliant member 25 isurged downward to move tips 26 into contact with pads 24. An aluminumoxide layer on pad 24 is broken by sharp tip 26 in order to makeelectrical contact between tip 24 and the aluminum metal of pad 24. Therigidity of small cantilever beams, however, is generally insufficientto apply the force to a tip that is necessary to cause it to breakthrough an aluminum oxide layer on a contact pad, without an externalmeans of applying force to the cantilever. Cantilever beams of glass,silicon, ceramic material, and tungsten have also been tried in variousconfigurations, without success in providing burn-in probes ofsufficient force and flexibility.

A flexible membrane probe is described in Flexible Contact Probe, IBMTechnical Disclosure Bulletin, October 1972, page 1513 as shown in FIG.2A. A flexible dielectric film 32 includes terminals 33 that are suitedto making electrical contact with pads on integrated circuits. Terminals33 are connected to test electronics by means of flexible wires 34attached to contact pads 35 on terminals 33. Probes fabricated on aflexible polyimide sheet were described in the Proceedings of the IEEEInternational Test Conference (1988) by Leslie et al. The flexible sheetallows a limited amount of vertical motion to accommodate variations inheight of bond pads on integrated circuits on a wafer under test.Membrane probes such as that described by Leslie et al provideconnections to integrated circuit chips for high performance testing.However, dimensional stability of the membrane is not sufficient toallow contacts to pads on a full wafer during a burn-in temperaturecycle.

Fabrication of the contacts on a thin silicon dioxide membrane asdescribed in U.S. Pat. No. 5,225,771 is shown in FIG. 2B. A silicondioxide membrane 40 has better dimensional stability than polyimide,thereby somewhat ameliorating the dimensional stability problem ofmating to contact pads on a wafer under burn-in test. Probe tips 41 areconnected by vias 44 through membrane 40 to circuit traces 45 that arelinked to an additional layer of circuitry 42 above a dielectric film43. However, limited vertical compliance of the test probes on silicondioxide membrane 40 renders use of such probe arrays unreliable for usein burn-in of devices on a semiconductor wafer.

Fabrication of an array of burn-in probes on a semiconductor wafer isdescribed in U.S. Pat. No. 4,585,991, especially as illustrated in FIGS.3A and 3B showing a top plan view and a sectional view respectively.Probe 51 is a pyramid attached to semiconductor wafer substrate 52 byarms 54. Material 53 is removed from the semiconductor wafer 52 in orderto mechanically isolate the probe 51. A probe as in FIG. 3A provides alimited vertical movement but it does not allow space on the substratefor wiring needed to connect an array of probes to test electronicsrequired for dynamic burn-in.

Another marginally successful approach to providing flexible probes todevice contact pads involves the use of flexible wires or posts toconnect the test circuitry to the pads. A flexible probe is described inU.S. Pat. No. 5,977,787 as shown in FIG. 4A. There, probe 60 is abuckling beam, earlier generally described in U.S. Pat. No. 3,806,801.Probe 60 is adapted for use in burn-in of devices on a wafer. Probe 60is held by guides 61 and 62, that have a coefficient of expansionsimilar to that of the wafer being tested. The probe tip 63 is offset bya small distance 60 to provide a definite modality of deflection forbeam 60. Although buckling beams are well suited to testing individualintegrated circuit chips, they are too expensive to be used for waferburn-in where thousands of contacts are required. Further, electricalperformance of buckling beam probes is limited because of the lengthrequired for adequate flexure of the beam.

Another approach using flexible posts as disclosed in U.S. Pat. No.5,513,430 is shown in FIG. 4B. FIG. 4b shows flexible probes in the formof posts 66 that are able to bend in response to force on probe tip 67.Posts 66 are formed at an angle to a substrate 69 in order to allow themto flex vertically in response to a force on tip 67 from mating contactpads. Posts 66 have a taper 65 from the base terminal 68 to tip 67 inorder to facilitate flexure.

Yet another approach using flexible wires and posts as disclosed in U.S.Pat. No. 5,878,486 is shown in FIG. 4C. The probe shown in FIG. 4Ccomprises a probe tip 72 on a spring wire 71 that is bent to a specificshape in order to facilitate flexure. Wire 71 is joined to substrate 74by a conventional wire bond 73. Probes of the type shown in FIG. 4Crequire a long spring length to achieve the contact force and compliancyneeded for wafer burn-in. Additionally, such probes that use individualwires are too expensive for use in wafer burn-in where many thousands ofprobes are required for each wafer.

Further approaches to providing flexible probes involve the use ofcompliant layers interposed between a test head and a device beingtested, such that terminals on the test head are electrically connectedto mating contact pads on the device. The electrical connector describedin U.S. Pat. No. 3,795,037 utilizes flexible conductors embedded in anelastomer material to make connections between mating pairs ofconductive lands that are pressed into contact with the top and bottomsurfaces of the electrical connector. Many variations of flexibleconductors are known including slanted wires, conductive filledpolymers, plated posts and other conductive means in elastomericmaterial in order to form compliant interposer layers.

The approaches listed above, however, and other attempts have beenunsuccessful in providing a high performance probe that allowseconomical burn-in and speed test of microelectronic devices on a waferbefore the wafer is diced into discrete chips.

SUMMARY OF THE INVENTION

In accordance with the present invention, a small compliant probe isdisclosed that includes a conductive tip, which is positioned on asupporting surface in a manner that allows a tip on the probe to moveflexibly with respect to the supporting surface. In a preferredembodiment, the probe tip moves vertically in response to the force of amating contact pad as it is biased against the tip. Mechanicalcompliance of the probe of the present invention allows electricalcontact to be made reliably between the probe and a correspondingcontact pad on a microelectronic device, where the mechanical complianceaccommodates variations in height of the contact pad.

It is an object of the present invention to provide a method and meansfor making electrical connection to contact pads on microelectronicdevices on an undiced wafer in order to burn-in the devices before theyare diced into separate chips. Compliant probes according to theinvention allow reliable electrical connections to be madesimultaneously to all of the contact pads arrayed on the surface of awafer so that microelectronic devices on the wafer can be burned-ineconomically.

Another object of the present invention is to provide a fixture forburn-in of microelectronic devices on undiced wafers. The fixtureelectrically connects contact pads on each device to drive circuitrythat supplies electrical signals to the device as required duringdynamic burn-in at high temperature. Electrical signals and power aresupplied to all of the chips on a wafer simultaneously. Mechanicalcompliance of probes in the fixture accommodates variations in height ofthe contact pads and in the probe tips such that each probe tip remainsin contact with its mating contact pad throughout the temperature cycleof the burn-in process.

Yet another object of the preferred embodiment of the present inventionis to provide an electrical probe card that allows high speed testing ofunpackaged microelectronic devices. Small, compliant probes as taughtherein are used to make temporary connections to corresponding pads on adevice in order to apply electrical test signals to that device and tomeasure electrical signals from that device. The small size of thecompliant probe allows high speed electrical signals to be passed to andfrom the device without losses due to excessive inductance orcapacitance associated with wire probes as used in the prior art.

A further object of the present invention is to provide a means forburning-in, testing and operating microelectronic devices whereelectrical contacts on the device are disposed in an area array over asurface of the device. Small, compliant probes as taught in thisdisclosure are used to make reliable electrical connections to contactson the device, where the contacts are arranged in an area array.Mechanical compliance allows the tip of each probe to maintainelectrical contact with a mating contact on the device notwithstandingvariations in the height of contacts on the device both at roomtemperature and at the operating temperature range of the device.

Another object of a preferred embodiment of the present invention is toprovide a small socket for connecting integrated circuit chips toelectrical circuits for purposes of burn-in, test and operation of thechip. The small size of each probe contact in the socket allowshigh-speed operation of a chip mounted in the socket. Mechanicalcompliance of the probes as taught herein enables reliable electricalconnections to be made to a rigid chip with minimal or no packaging.Compliant probes according to the present invention allow constructionof small, economical sockets for chip scale packages and for flip-hips.

The probe disclosed herein is significantly improved over conventionalcantilever probes in that it provides a greater range of compliantmotion of the probe tip for any given probe force and probe size. Aconventional cantilever probe is limited in the range of motion itprovides in response to a given force before the elastic limit of theprobe material is reached. The maximum mechanical stress in cantileverprobes is concentrated on the surface of the cantilever material at thepoint of flexure. The present invention provides a greater range ofmotion for a given spring material and probe force, before reaching theelastic limit of that material.

The invention increases manufacturing efficiency for microelectronicdevices by reliably providing test and burn-in functions at the waferlevel, while at the same time reducing the size of the test fixture. Themechanically compliant probe of the present invention provides a largerange of motion relative to the size of the probe. This range of motionis important in making connections to a device with contact pads thatare not substantially in the same plane. The compliant probe tip of thepresent invention moves flexibly to accommodate differences in theheight of mating contact pads while maintaining sufficient force of theprobe tip on the contact pad to assure reliable electrical contact therebetween.

These as well as other objects of the invention are met by providing amechanically compliant electrical probe. In a preferred embodiment, aprobe tip is disposed on an elongated thin strip of material that issupported at both ends and wherein the tip is positioned at apredetermined distance from a center line connecting the centers of thesupports at each end of the strip. The probe tip thus supported movescompliantly in a vertical direction by torsional and bending flexure ofthe thin strip of material.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features characteristic of this invention are set forth in theappended claims. The invention itself as well as other features andadvantages thereof are best understood by reference to the detaileddescription that follows, read in conjunction with the accompanyingdrawings wherein:

FIG. 1 shows a sectional view of a cantilever probe of the prior art;

FIGS. 2A and 2B show cross sectional views of flexible membrane probesof the prior art;

FIGS. 3A and 3B show views of a probe fabricated on a silicon wafer ofthe prior art where FIG. 3A shows a plan top view of the probe and FIG.3B shows a sectional view of the probe;

FIGS. 4A to 4C show flexible post probes of the prior art;

FIG. 5 shows a view of a compliant probe in accordance with the presentinvention;

FIG. 6 shows a view of an alternate configuration of a compliant probein accordance with the present invention;

FIGS. 7A to 7C show an embodiment of a compliant probe where FIG. 7A isa top plan view, FIG. 7B is a sectional view of the probe at rest, andFIG. 7C is a sectional view of the probe when acted upon by force F;

FIG. 8A shows a view of an embodiment of a compliant probe as its probetip is acted upon by a force F directed vertically;

FIG. 8B shows the deflection of the probe tip of FIG. 8A as a functionof the force acting on the probe tip;

FIGS. 9A to 9C show an embodiment of a compliant probe where FIG. 9A isa top plan view, FIG. 9B is a sectional view of the probe at rest, andFIG. 9C is a sectional view of the probe when acted upon by force F;

FIG. 10 shows a view of an embodiment of a compliant probe and itsconnection circuit;

FIG. 11 shows a view of an embodiment of a compliant probe with a groundplane;

FIGS. 12A to 12C show an embodiment of a compliant probe with itscircuit connection where FIG. 12A is a top plan view, FIG. 12B is asectional view of the probe at rest and FIG. 12C is a sectional view ofthe probe when acted upon by force F;

FIGS. 13A to 13C show top plan views of alternative designs forcompliant probes according to the present invention;

FIGS. 14A to 14D show top plan views of alternative designs forcompliant probes according to the present invention;

FIG. 15A shows a connector for wafer level burn-in of devices with areaarray contacts;

FIG. 15B shows a top plan view of a selected area of the connector ofFIG. 15A for devices with area array contacts;

FIG. 16A shows a probe card for wafer level testing of devices with areaarray contacts;

FIG. 16B shows a top plan view of a selected area of the probe card ofFIG. 16A for devices with area array contacts;

FIG. 17A shows a socket for operating microelectronic devices with areaarray contacts;

FIG. 17B shows a top plan view of a selected area of the socket of FIG.17A for devices with area array contacts;

FIGS. 18A to 18D show probe tips for use in compliant probe structuresaccording to the present invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior art probes are illustrated in FIGS. 1-4. In accordance with theprinciples of the invention, a first preferred embodiment of a compliantprobe is shown in FIG. 5. A probe is disclosed that makes reliableelectrical connection to contact pads (not shown) on a microelectronicdevices such as integrated circuits (ICs), flip-chips, passive devices,and chip scale packages. The probe provides flexible vertical motion ofa probe tip 81 in response to a force on the tip. Thus, as a contact padis urged into contact with probe tip 81, mechanical compliance of theinvention's structure allows the tip to make contact with the matingcontact pad at a force sufficient for probe tip 81 to penetrate aninsulating oxide film on the pad. Mechanical compliance of the probeaccommodates differences in height of the contact pads in a region ofthe microelectronic device while providing sufficient force on eachprobe tip to assure a reliable electrical connection between the tip andthe corresponding contact pad. Further, mechanical compliance of the padis necessary to allow the tip to maintain a connection to thecorresponding pad during a test or burn-in cycle where thermal expansionmay cause warping of the device and of the probe support.

In FIG. 5, probe tip 81 is supported on a lateral extension arm ofconductive material 82 that is attached to an elongated flexible strip83 of conductive material. Elongated flexible strip 83 is supported ateach end by posts 85 that are joined to terminals 84 on elongated strip83. Probe tip 81 moves flexibly in response to a force appliedvertically to tip 81. Vertical movement of tip 81 depresses arm 82 andtorsionally flexes strip 83 thereby impressing a restoring force on tip81.

In the compliant probe shown in FIG. 5, posts 85 are supported onsubstrate 89 by pads 86 which are connected electrically to a circuittrace 87 which is connected in turn to electrical circuitry in substrate89 by means of a via contact 88 that links circuit 87 on the surface tocircuit layers in substrate 89. By the series of links described above,probe tip 81 is connected electrically to circuits in the substrate 89that operate a device that is connected to the probe. In demandingapplications such as burn-in, substrate 89, in a preferred embodiment,is made of silicon or a low expansion ceramic material in order toachieve dimensional stability over a wide temperature range such asthose used in burn-in, where a temperature cycle may go from 25° C. to150° C. or greater.

For operation at high frequencies, the electrical links from probe 81 tovia contact 88 are arranged to minimize the inductance of the connectionto probe tip 81. The inductive loop is minimized by locating via contact88 under probe tip 81. While via contact 88 cannot always be so ideallylocated, the distance between tip 81 and via contact 88 is small inthose applications that require high frequency operation.

FIG. 6 shows a second embodiment for the compliant probe of the presentinvention where conductive arm 82 of FIG. 5 is replaced by a curved or“V” shaped segment 92 of an elongated thin sheet 93 in FIG. 6. Terminals94 on each end of elongated thin sheet 93 are joined to posts 95, whichin turn rest on pads 96 on a substrate 99. Electrical connection to tip91 is made through sheet 93 to pads 94 joined to posts 95 resting onpads 96 that are connected by a circuit trace 97 connected to electricalcircuits in substrate 99 by means of a via contact 98.

As seen in FIG.6, probe tip 91 is supported on the curved portion 92 ofsheet 93 such that the center of probe tip 91 is located at a distancefrom an imaginary dashed line 100 between posts 95 at each end ofelongated thin sheet 93. An initial vertical force on probe tip 91produces a torque about an axis represented by line 100. The torquecauses a torsional flexure of elongated thin sheet 93, which produces acounter force acting to oppose the initial force on tip 91.

FIG. 7A shows the plan view from above of a first embodiment of thecompliant probe of the type illustrated in FIG. 6. A flexible elongatedstrip 103, in a preferred embodiment is made of a sheet of metal shapedto include a lateral extension 102 at the midpoint of strip 103 andcontact pads 104 at each end of strip 103. The electrically conductivematerial of strip 103 is chosen to exhibit high yield strength andmoderate elongation at ultimate failure. Metals chosen from the groupconsisting of beryllium-copper alloys, columbium, cupro-nickel,molybdenum, nickel, nickel-titanium, stainless steel, titanium, andalloys thereof are suitable. Applicant has determined one suitable metalis beryllium-copper alloy ASTM Spec. No. B534, with a yield strength of550 mega-Pascals. Another suitable metal is titanium alloy Ti, 8 Al, 1Mo, 1 V, with a yield strength of 910 mega-Pascals.

A probe tip 101 shown in FIG. 7A is supported on extension arm 102 suchthat probe tip 101 depresses vertically toward a substrate 109 inresponse to a vertical force F. The action of arm 102 and probe tip 101is shown in sectional views of FIGS. 7B and 7C. A force F applied toprobe tip 101 exerts a torque on strip 103, twisting the strip andallowing arm 102 to depress toward substrate 109. As seen in thesectional view in FIG. 7C, the vertical motion of probe tip 101 is dueto the action of both beam deflection and torsional bending of strip103.

Probe tip 101 in a preferred embodiment is a pyramid formed byreplication of an etch pit formed in a (100) silicon surface bywell-known processes. The tip angle of 54.75° is determined by the (111)crystallographic planes in silicon. The material of the tip is tungsten,which forms a sharp, hard tip that is able to break through aluminumoxide layers on aluminum contact pads typically used on semiconductor ICdevices. Applicant has determined that other materials suitable formaking hard probe tips may be selected from a group consisting ofmolybdenum, nickel alloys, osmium, Paliney 7, rhodium, rhenium,titanium, tungsten and alloys thereof.

Fabrication of sharp probe tips by replication of etch pits in siliconis well known in the field of electrical contacts and is well describedin a publication in 1973 by D. A. Kiewit in Reviews of ScientificInstruments, Vol. 44, pages 1741-1742. Kiewit describes formation ofprobe tips that are made by replication of etch pits in silicon bydepositing nickel-boron alloy into the pit, and then removing thesilicon matrix material to expose the pyramid. Kiewit formed pyramidaletch pits in silicon (100) surfaces by treating the surface with boilinghydrazine hydrate. Any method now known or hereafter developed formaking appropriate probe tips is suitable.

Strip 103 is supported above substrate 109 by posts 105 that are joinedto contact pads 104 at each end of strip 103. Post 105 is formed of anelectrodeposited metal preferably chosen from the group consisting ofhard copper, nickel, cupro-nickel alloys, and hard gold. Electricalconnection of probe tip 101 to circuits for testing integrated circuitsis made by conduction through arm 102, strip 103, contacts 104, posts105, contact pads 106, a conductor 107 and a via contact 108. Theelectrical circuit from via contact 108 to probe 101 is configured toform as small a loop as possible in order to reduce inductance andthereby allow operation at the highest frequencies or data rates.

FIGS. 8A and 8B illustrate in greater detail the operation of apreferred embodiment configuration in which a probe tip 111 is supportedby a lateral extension arm 112 on a thin strip of material 113 heldbetween two support posts 115. Force F depresses tip 111 by a deflectionof δ_(T) in the vertical direction.

The total deflection δ_(T) of tip 111 shown in FIG. 8B is a sum of thebeam bending component δ_(D) and the torsional deflection component.FIG. 8B shows the total deflection δ_(T) in microns caused by a force Fin grams acting vertically on probe tip 111. For this study, strip 113,in a preferred embodiment, is made of molybdenum with a thickness of 25microns, a width of 25 microns, and a length of 200 microns. Arm 112, ina preferred embodiment, is 100 microns long from the centerline of strip113 to the probe tip, as measured in the plane of the strip.

A detailed view of the second preferred embodiment of the compliantprobe of the present invention is shown in a top plan view of FIG. 9A.Probe tip 121 is supported on a “V” shaped extension 122 of elongatedthin sheet 123. In a preferred embodiment, extension 122 supports tip121 at a position to one side of an imaginary line connecting thecenters of posts 125 that support each end of elongated thin sheet 123.Extension 122 is thicker than the main body of elongated thin sheet 123in order to prevent distortion to the shape of the extension by appliedforce F.

A probe tip 121 declines vertically toward a substrate 129 in responseto a vertical force F applied to tip 121. The deflection of extension122 and probe tip 121 is shown in the sectional views in FIGS. 9B and9C. A force F applied to probe tip 121 exerts a torque on sheet 123,thereby twisting sheet 123 and allowing extension 122 to depress towardsubstrate 129. As seen in the sectional view in FIG. 9C, the verticalmotion of probe tip 121 is due to both beam deflection and torsionalbending of elongated thin sheet 123.

Sheet 123 is supported above substrate 129 by posts 125 joined tocontact pads 124 at each end of sheet 123. Posts 125 are rigid metalposts. Electrical connection of probe tip 121 to test circuits is madeby conduction through arm 122, sheet 123, contact pads 124, posts 125,contact pads 126, a circuit trace 127, and a via 128. The electricalcircuit from via 128 to probe 121 is configured to form as small a loopas possible in order to reduce inductance and thereby allow operation atoptimum electrical performance.

FIGS. 10 and 11 show additional embodiments of the compliant probe ofthe present invention where the function of the extension arm and thethin elongated sheet are combined into one structure. A third embodimentis shown in FIG. 10 wherein a probe tip 131 is disposed on a curvedelongated thin sheet 133 such that the center of probe tip 131 islocated at a predetermined distance from an imaginary line connectingthe centers of support posts 135 at each end of curved sheet 133. Curvedelongated thin sheet 133 flexes torsionally and bends in response to aforce applied vertically to probe tip 131. The torsional twist is due tothe torque generated by the force applied at a distance from thecenterline of support posts 135. The amount of torsional flexurerelative to beam bending flexure is dependent upon the offset of probetip 131 from the centerline as a fraction of the length of curved sheet133. Depending upon the dimensions of the device being tested and thematerial properties of curved sheet 133. The offset is preferablybetween 0.05 to 0.5 times the length of curved sheet 133.

The probe of FIG. 10 includes curved sheet 133 that supports probe tip131 which is offset from the centerline of support posts 135. Electricalconnection to the probe tip 131 is made through curved sheet 133 tocontact terminals 134. Terminal 134 is in turn joined to posts 135 thatrest on contact pads 136 connected to a circuit trace 137 that is linkedby a via 138 to test circuits in substrate 139. Via 138 is positionedproximal to the probe tip in order to minimize inductance of the linkconnecting the test circuits to probe tip 131.

A fourth embodiment of the compliant probe that incorporates a groundplane shield is shown in FIG. 11. The probe of FIG. 11 includes a curvedsheet 143 that supports a probe tip 141 located at a position that isoffset from the centerline of support posts 145. Electrical connectionto the probe tip 141 is through sheet 143 to contact pads 144. In turn,contact pads 144 are joined to posts 145 that rest on terminals 146connected to a circuit trace 147 that is linked by a via 148 to testcircuits in a substrate 149. A ground layer 140 underlies probe tip 141and shields the probe electrically in order to achieve higherperformance.

A detailed view of the embodiment illustrated in FIG. 10 is shown inFIGS. 12A-12C. A top plan view of FIG. 12A shows a representativeconfiguration of the third preferred embodiment, where a tip 151 issupported on the midpoint of a “V” shaped flat sheet 153 of springmaterial. The “V” shaped sheet 153 is supported by terminals 154disposed at each end of the sheet. The sheet in this embodiment is madeof titanium alloy Ti, Al 8, V 4, although other high strength orsuperplastic materials would serve as well. The thickness of sheet 153is between 10 and 75 microns, and more preferably the thickness isbetween 20 and 50 microns. The width of the narrowest section of eacharm 153 is between 20 and 200 microns and more preferably the width isbetween 35 and 75 microns. The distance between the centroid of post 155at a first end of sheet 153 and the centroid of post 155 at the secondend of sheet 153 is about 200 to 1000 microns in length, and morepreferably the center to center spacing is 250 to 750 microns.

Response of the probe to a force F on probe tip 151 is illustrated inFIGS. 12B and 12C, showing a sectional view of the compliant probebefore and after the application of force F. As shown in FIG. 12C, aforce F on probe tip 151 deflects the thin curved sheet 153 downwardtowards a substrate 159. Thin curved sheet 153 is both bent and twistedtorsionally by the deflection. Torsional and bending deflection of sheet153 generates a counter force that opposes further deflection of tip 151as it is acted upon by force F.

Probe tip 151 is connected to electrical circuitry by sheet 153 that issupported by posts 155 joined to contact pads 154 of sheet 153. Posts155 rest upon terminals 156 positioned on substrate 159, where terminals156 connect to a circuit trace 157. Circuit trace 157 is joined toelectrical circuitry in substrate 159 by a conductive via 158.Optionally, a ground plane (not shown) may be inserted between the probetip 151 and circuitry in substrate 159 in order to shield tip 151 fromsignals in adjacent circuit traces in substrate 159.

Variations in the design of the sheet spring in the compliant probe aremade to accommodate the test requirements of specific microelectronicsdevices. Several designs are illustrated in FIGS. 13A to 13C. In eachcase, however, the probe tip is positioned off of the axis determined byan imaginary line through the centroids of the posts supporting thesheet spring at a first and at a second end.

FIG. 13A illustrates a preferred embodiment of a design for a probe 160where a probe tip 161 is supported at the apex of a “V” shaped segment162 of a sheet spring 163. Segment 162 is positioned toward one end ofspring 163 to allow nesting of springs necessary to achieve a closespacing between probe tips. Posts 165 and 167 are positioned in astaggered pattern to allow close spacing of the individual probes.Correspondingly, contact pads 164 and 166 on opposing ends of sheetspring 163 are matched to posts 165 and 167, respectively.

FIG. 13B illustrates a preferred embodiment of a design for a probe 170where a probe tip 171 is supported at the apex of a “V” shaped segment172 of a sheet spring 173. Segment 172 is positioned toward the end ofsheet spring 173 to allow nesting of springs necessary to achieve aclose spacing between probe tips. Posts 175 and 177 are positioned in astaggered pattern to allow close spacing of the individual probes.Correspondingly, contact pads 174 and 176 on opposing ends of sheetspring 173 are matched to posts 175 and 177, respectively.

FIG. 13C illustrates a preferred embodiment of a design for a compliantprobe 180 wherein a probe tip 181 is supported at the midpoint of acurved sheet spring 182. Curved spring 182 is shaped to allow nesting ofsprings necessary to achieve a close spacing between probe tips. Probetip 181 is offset from the centerline between centroids of the posts 185disposed at each end of spring 182. Contact pads 184 at each end ofsheet spring 182 are joined to posts 185.

Alternatively, Applicant has determined that unsymmetricalconfigurations of the compliant probes shown in FIGS. 14A to 14D providecapabilities needed for specific testing and burn-in applications.Unsymmetrical configurations facilitate probing of contact pads inconstrained spaces, in corners, and on chips with a small pad pitch.Further, an optional ground contact allows ground shielding to beincorporated into the probe structure.

Compliant probe 190 shown in FIG. 14A utilizes a post 195 to support thefirst end of an elongated thin sheet 192. The second end of elongatedthin sheet 192 is supported by a post 195. An additional post 197 isused to stabilize the structure against lateral forces. Additional post197 is also used to make electrical contact with a ground plane 199incorporated into the probe. Post 197 is joined to ground plane 199 atcontact pad 196. Thin sheet 192 is connected to posts 195 that arejoined to sheet 192 by contact pads 194.

Thin elongated sheet 192 supports a probe tip 191 disposed at a positionthat is offset from the central axis 198 of probe 190. The central axisis an imaginary line that connects the centroid of posts 195 and 197that support the first end of sheet 192 with the centroid of posts 195that supports the second end of member 192. Force applied to probe tip191 creates a torque about central axis 198 that causes member 192 tobend and to twist torsionally.

In FIG. 14B, a compliant probe 200 includes an elongated sheet springwith a short segment 202 supported by post 207 and a long segment 203supported by a post 205. Contact pads 204 and 206 join the sheet toposts 205 and 207, respectively. The sheet spring supports a probe tip201 that is disposed between segment 202 and segment 203 at a positionthat is offset from a centerline 208 of probe 200. Centerline 208 is animaginary line that connects the centroid of post 205 with the centroidof post 207. A force applied to probe tip 201 creates a torque aboutcenterline 208 that causes arms 202 and 203 to bend and to twisttorsionally.

In FIG. 14C, a compliant probe 210 includes a sheet spring with a shortsegment 212 supported by a contact pad 216 joined to a post 217, and along segment 213 supported by a contact pad 214 joined to a post 215.The sheet spring supports a probe tip 211 that is disposed betweensegment 212 and segment 213 at a position that is offset from acenterline 218 of probe 210. Centerline 218 is an imaginary line thatconnects the centroid of post 215 with the centroid of post 217. Forceapplied to probe tip 211 creates a torque about centerline 218 thatcauses thin sheet segments 212 and 213 to bend and to twist torsionally.

A compliant probe 220 in FIG. 14D includes a sheet spring with a shortsegment 222 with a contact pad 226 joined to a post 227, and with a longsegment 223 with a contact pad 224 joined to a post 225. The sheetspring supports a probe tip 221 that is disposed between segments 222and 223 at a position that is offset from a centerline 228 of probe 220.Centerline 228 is an imaginary line that connects the centroid of post225 with the centroid of post 227. Force applied to probe tip 221creates a torque about centerline 228 that causes arms 222 and 223 tobend and to twist torsionally, thereby generating a counterforce thatlimits further deflection of probe tip 221.

Compliant probes according to teachings of this invention can be usedfor burn-in of wafers containing integrated circuits and othermicroelectronic devices. A wafer connector 230 shown in FIG. 15Aincorporates probes 232, configured, for example, to the preferredembodiment illustrated in FIG. 10, on a surface of a silicon substrate231. Each of probes 232 is connected to terminals 233 on contactor 230by circuit traces 234 in silicon substrate 231. In this example, siliconis used as the material for substrate 231 in order to provide a thermalexpansion coefficient that is matched to that of a silicon wafercontaining integrated circuits under burn-in test.

In performing burn-in, connector 230 is aligned to a wafer under testand then held with a mechanical clamping means such that each probe ofthe connector is biased against a mating contact pad on the wafer with aforce sufficient to assure reliable contact. Applicant has determinedthat to contact standard aluminum pads, a force of from 5 to 10 grams issufficient to assure contact. The assembly is then heated to the burn-intemperature, typically 125° C. to 150° C. Electrical stimuli are appliedto each integrated circuit to exercise the circuit and accomplishdynamic burn-in at the bum-in temperature.

FIG. 15B shows a portion of the probes that are disposed on the surfaceof connector 230. The probe tips are arranged in an area array that ismatched to an area array of contact pads on flip-chips being tested.Each probe tip 241 is positioned to mate with a corresponding contactpad on the flip-chip. The dimensions of probe 232 are compatible with agrid pitch spacing of between 150 microns and 500 microns currentlyutilized for flip-chips. Probes 232 are arranged in a nested patternthat allows each probe to fit the space available. In a preferredembodiment, additional non-functional probes are added to the array toprovide support to the wafer under test in local regions where theaverage density of contact pads on the wafer is low. Any requireddimensions are suitable for the invention.

Probe tips 241 of probe 232 provide a hard surface for the purpose ofbreaking through any oxide on the aluminum bond pads on the wafer undertest. Probe tip 241 is disposed at the apex of a “V” shaped elongatedthin sheet 242 that is supported by posts 245 joined to contact pads 244at each end of sheet 242.

Compliant probes according to the teachings of this invention provide ameans to test high-speed integrated circuits because of the low self andmutual inductance of each probe. A probe card 249 incorporatingcompliant probes is shown in FIG. 16A. Probes 240 are disposed in anarea array pattern on a substrate 248 suitable for testing flip-chipswith area array contact pads. Each probe 240 is connected electricallyto terminals 247 on probe card 249 by circuit trace means 246incorporated in substrate 248. Substrate 248 is preferably made of adimensionally stable base such as alumina ceramic material, on whichcircuit traces are disposed between layers of polyimide dielectricmaterial.

FIG. 16B shows an array of compliant probes 240 configured according tothe teachings of the invention illustrated in FIG. 5, for example. Aprobe tip 241 is disposed at the end of extension arm 243 at themidpoint of elongated sheet spring 242. Support posts 244 are joined tocontact pads 245 at each end of elongated sheet spring 242 so that probetip 241 on arm 243 is moveably compliant in a vertical direction.

A chip socket shown in FIG. 17A provides a demountable means fortesting, burning-in and operating flip-chips. Flip-chip 261 is held bypositioning means 262 such that each contact pad on flip-chip 261 ismated with a corresponding probe 250 on the surface of socket substrate258. Each probe 250 is connected electrically with terminals 257 onsocket substrate 258 by circuit trace means 256. Electrical signalssuitable for operating flip-chip 261 are directed to the socket byinterconnection means 263 from electronic circuitry means 264. Cable 265connects the electronic circuitry 264 to the system for burn-in, test oroperation of flip chip 261.

FIG. 17B shows a portion of the array of compliant probes 240 in thesocket of FIG. 17A. A probe tip 251 is disposed at the end of anextension arm 253 attached to the mid-point of an elongated sheet spring252. Support posts 255 are joined to contact pads 254 at each end ofelongated sheet spring 252.

Preferred embodiments of probe tips shown in FIGS. 18A to 18D areconfigured for specific applications in testing and burn-in. These probetips and others are well known in the integrated circuit industry, andthe examples presented here are representative of the many types ofprobe tips that are available. Methods of fabrication are well known toskilled practitioners in the art of manufacturing electrical contacts.

A probe tip shown in FIG. 18A is preferred for probing aluminum bondpads on integrated circuits. A sharp apex 273 is suited to breakingthrough the oxide layer on aluminum bond pads. A pyramid 272 is formedby replication of an etch pit in a (100) silicon surface. Pyramid 272 issupported on a sheet spring 271. Apex 273 of pyramid 272 is sharplydefined with an included angle of 54.75° between opposite faces. A hardmaterial is used for probe tip 272, where the material is preferablyselected from the group consisting of molybdenum, nickel, osmium,Paliney 7, rhodium, rhenium, titanium, tungsten, and their alloys. Inprobing soft contacts, materials such as osmium, rhodium, and tungstenare preferred because they react slowly with solders and other softmaterials.

A probe tip shown in FIG. 18B is suited for contacting noble metalcontact pads. A thin disk 277 is supported on a metal post 276 disposedon a sheet spring 275. Post 276 is undercut by chemical etching toexpose edges of disk 277. Thin disk 277 is made of an inert metalpreferably selected from the group consisting of gold, Paliney 7,Platinum, Rhodium, and their alloys.

A probe tip shown in FIG. 18C is suited to contacting solder and othersoft materials. A rounded metal tip 281 is supported on a metal post 282that is disposed on a sheet spring 280. Rounded metal tip 281 can beshaped by flash laser melting of a high temperature material to reflowinto the shape of a spherical section. Materials suitable for roundedmetal tip 281 include nickel, platinum, rhodium, cupro-nickel alloys,beryllium-copper alloys, and Paliney 7.

A probe tip shown in FIG. 18D is suited to contacting small contact padsand pads that are spaced closely together. A probe tip 287 with a topedge 286 is disposed on the top surface of a sheet spring 285. Probe tip287 is preferably formed by plating the edge of a sacrificial materialand then removing that material to leave a thin sheet of metal 287projecting vertically from sheet spring 285.

Although several preferred embodiments of the invention have beendescribed, numerous modifications and alternatives thereto would beapparent to one having ordinary skill in the art without departing fromthe spirit and scope of the invention as set forth in the followingclaims.

What is claimed:
 1. A probe for making electrical connection to contactpads on microelectronic devices, said probe comprising: (a) a sheet ofconductive material with a main body having a first and a second end,said main body acting as a torsional spring; (b) a rigid substrate witha top and a bottom surface; (c) at least one first rigid electricallyconductive post supporting said first end of said main body above saidtop surface of said substrate; (d) at least one second rigidelectrically conductive post supporting said second end of said mainbody above said top surface of said substrate; (e) an arm of said sheetextending away from said main body; and (f) an electrically conductivetip structure with a base disposed on said top surface of said armdistal from said main body and a tip point extending away from said topsurface of said arm, wherein a bias of said conductive tip toward saidsubstrate causes substantial torsional distortion of said main body ofsaid sheet.
 2. The probe of claim 1 wherein said sheet is asubstantially flat metal foil with a longest dimension of between 150micrometer and 1500 micrometers, and with a thickness of between 10micrometers and 75 micrometers.
 3. The probe of claim 1 wherein said armis of a V shape, with a thickness greater than thickness of said mainbody.
 4. The probe of claim 1 wherein said conductive material is ametal selected from the group consisting of beryllium-copper, columbium,cupro-nickel, molybdenum, nickel, nickel-titanium, stainless steel,titanium, tungsten, and alloys thereof.
 5. The probe of claim 1 whereinsaid conductive tip is of a hard metal selected from the groupconsisting of chromium, nickel, osmium, Paliney 7, rhenium, rhodium,titanium, tungsten, and alloys thereof.
 6. The probe of claim 1 furtherincluding a ground plane shield on a region of said substrate thatunderlies said electrically conductive tip.
 7. The probe of claim 1wherein said electrically conductive tip comprises a pyramid.
 8. Theprobe of claim 1 wherein said electrically conductive tip comprises acylindrical metal projection oriented substantially vertically abovesaid sheet.
 9. The probe of claim 1 wherein said electrically conductivetip comprises a spherical section of metal.
 10. The probe of claim 1wherein said rigid posts are electroplated metal.
 11. The probe of claim1 further including an elastic dielectric material disposed between saidsheet and said top surface of said substrate.
 12. The probe of claim 11wherein said elastic dielectric material is selected from the groupconsisting of silicone, fluorosilicone, fluorocarbon, and urethaneelastomer.
 13. A probe for making electrical connection to contacts onmicroelectronic circuits, said probe comprising: (a) a rigid substrate;(b) an elongated thin spring with a top and a bottom surface; (c) atleast one first rigid post supporting said spring at a first end abovesaid substrate; (d) at least one second rigid post supporting saidspring at a second end above said substrate; (e) an arm extendinglaterally from said spring to a distal end disposed at the mid pointbetween said first end and said second end; and (f) a probe tip disposedat said distal end of said arm such that a force applied vertically tosaid probe tip causes torsional flexing of said elongated thin spring.14. The probe of claim 13 further including a metal film ground plane onsaid substrate, said thin metal ground plane disposed under saidelectrically conductive tip.
 15. The probe of claim 13 wherein saidsheet comprises a planar sheet of metal patterned such that said patternis curved in a direction parallel to said thin sheet.
 16. The probe ofclaim 13 wherein said first post comprises a first metal post and saidsecond post comprises a second metal post.
 17. A probe for makingelectrical connections to contact pads on a microelectronic device, saidprobe comprising: (a) a rigid substrate; (b) a strip of electricallyconductive material with a first and a second end, said strip disposedabove said rigid substrate and acting as a torsional spring; (c) a rigidsupport at said first end and a rigid support at said second end of saidstrip; (d) an arm of said strip extending in a direction substantiallyperpendicular to an axis between said first end of said strip and saidsecond end of said strip; (e) an electrically conductive tip disposed ona distal end of said arm; (f) whereby a bias of said conductive tiptoward said substrate is operative to cause torsional flexure of saidthin strip.